1. Field of the Invention
The present invention relates to a method for fabricating improved electrodes. More particularly, the present invention relates to a method for fabricating electrodes suitable for use as part of a cathodic protection system in corrosive environments such as, for example, in shell and tube heat exchangers.
2. Description of the Prior Art
In general, in fabricating shell and tube heat exchangers, the shell and tubes are made from carbon steel and type 316 stainless steel is used for the sheets mounted inwardly and at either end of the shell for supporting the tubes.
Particularly, when brackish water is employed as the heat exchange medium in such exchangers, it has been observed that current flows from the more active carbon steel to the more passive stainless steel. In such a situation, iron within the carbon steel oxidizes to the ferrous ion with the loss of two electrons in accordance with the following equation: ##EQU1## The net result of this electro conduction is corrosion and wearing away of the carbon steel. Such corrosion occurs in those regions where the tubes pass through openings in the supporting sheets with the consequent wearing away of the sheet and the dislocation of the tubes.
In the past, attempts have been made to avoid or at least substantially minimize such corrosion by utilization of cathodic protection systems.
In one such system, titanium rods have been mounted in the dished ends of the shell and tube heat exchangers and electrically connected to the shell of the heat exchanger to provide a cathodic protection system. The titanium anodes have had platinum patches welded thereto for the purpose of current "values." Such titanium electrodes have served as the anode in the system with the inner walls of the dished ends of the exchanger serving as the cathode. Unfortunately, in commercial operation, it has been determined that the platinum patches have a relatively short life. While the mechanism for this is not fully understood it is believed that, in operation, the skin voltage on the titanium anode becomes more active than the titanium base in the weld area causing the weld to disintegrate and release the platinum patch into the dished ends of the heat exchanger.
The base titanium metal has an extremely positive "skin" voltage ranging anywhere from 4-12 volts and when the patch is welded on to this titanium the titanium weld area has a much more active "skin" voltage and is less corrosion resistant. Being less corrosion resistant, the weld area is easily corroded, permitting the platinum patch to be removed prematurely from the anode.